Transient axonal pathways develop during the ontogenesis of the central nervous system, but generally retract through maturation such that adult patterns of connectivity are formed. Our recent studies have identified a transient projection from the somatosensory areas of the cerebral cortex to the cerebellum in neonatal kittens. The proposed experiments will examine two hypothetical roles for transient cerebrocerebellar projections. First, cerebro-cerebellar axons may serve to organize and stabilize the termination of other mossy fibers, which are highly organized in the adult, into functionally appropriate areas of the cerebellum. To test this hypothesis small lesions will be made in the hindlimb area of the primary somatosensory cortex in newborn kittens. After the kittens mature the organization of afferent input from brain stem nuclei to hindlimb somatosensory areas of the cerebellum will be mapped by utilizing the retrograde transport of horseradish peroxidase (HRP). The disorganization of neurons projecting to those areas of the cerebellar cortex which did not receive a transient cerebro-cerebellar projection would indicate that cerebro-cerebellar axons serve to validate other cerebellar mossy fiber systems. Secondly, cerebro-cerebellar projections may provide a substrate for functional compensation after lesions early in development that destroy somatosensory input to the cerebellum. Lesions will be made in the neonate in either the middle cerebellar peduncle to interrupt the ponto-cerebellar system relaying somatosensory information from the neocortex, or the inferior cerebellar peduncle to disrupt somatosensory input from brain stem and spinal neurons. Both orthograde (autoradiographic technique) and retrograde (HRP technique) labeling methods will be used to demonstrate any persistance of cerebro-cerebellar axons in juvenile kittens. Their presence would prove that this normally transient pathway can persist after neonatal lesions and possibly could provide an alternative pathway through which the "developmentally injured" cerebellum could obtain sensory information. The results from these studies will provide much needed and important data concerning the function and plasticity of transient axonal pathways during the normal development of the central nervous system. They also will provide valuable data on the organization of mossy fibers early in development.